function x_star = FD(x0,eps)     
    global G b;
    gk = gradf1(x0);                       
    res = norm(gk);                      
    k = 0;
    while res > eps && k<5000          
        dk = -gk;                        
        ak = 1;                          
        f0 = func(x0);                    
        f1 = func(x0+ak*dk);              
        slope = dot(gk,dk);              
        while f1>f0+0.0001*ak*slope
            ak = ak/2;
            xk = x0+ak*dk;               
            f1 = func(xk);
        end                              
        k = k+1;
        x0 = xk;
        gk = gradf1(xk);
        res = norm(gk);
        
    end
    x_star = xk
    fprintf('FD k=%d时,精度为%f\n',k,res);
    g_f = gradf1(xk);
    f_min = func(xk);
    fprintf('FD min f(x)=%f \n',f_min);
end